Various types of riser tensioners have been devised for use in the oil and gas industry. These tensioners help to maintain a desired tension on a riser extending between a subsea oil well and a surface (e.g., floating) drilling or production platform. Ram-style riser tensioners are often used to provide tension to risers used in spar and tension leg platform (TLP) applications. Ram-style riser tensioners may also be used as wireline tensioners in applications with marine drilling risers. Ram-style tensioners include hydro-pneumatic cylinders used to maintain a nearly constant tension on production risers or drilling risers as the floating platform moves in the ocean due to waves, current, and other factors.
In conventional ram-style tensioners, the cylinders typically include a cylinder barrel and a rod barrel that are able to slide, sweep, or stroke relative to one another to lengthen or compress the cylinder. Seals are placed between the barrels at their ends to prevent high pressure fluid from escaping the cylinder, to lubricate and enable the barrels to sweep relative to each other. The hydro-pneumatic cylinders are often filled with hydraulic fluid or oil to keep the seals lubricated, while compressed air or nitrogen is used as a gas spring to maintain tension in the riser. The cylinders are typically connected to an external gas accumulator, which is sized to provide a spring constant within a range that is conducive to the riser design.
Some applications for ram-style riser tensioners (e.g., spar and marine drilling riser applications) tend to produce long strokes on the cylinder compared to other applications (e.g., TLP applications). Thus, spar and marine drilling riser tensioners often utilize large sources of compressed air or nitrogen to maintain a sufficiently soft system during the long cylinder strokes. The swept volume in these cylinders can be quite large, often exceeding 200 gallons. Large volumes of hydraulic fluid are desirable for maintaining the seals on these long-stroking cylinders, since the fluid volume must have space to flow as the cylinder compresses. This fluid is generally contained within the cylinder and/or an accumulator, and large accumulators are often used to provide this volume of fluid. Unfortunately, large accumulators can take up a large amount of deck space and add undesirable weight to the cylinder assembly.